1. Field of the Invention
The present invention relates to a radiation imaging device having a scintillator for converting radiation into light, and a radiation imaging system and method using the radiation imaging device.
2. Description Related to the Prior Art
A radiation imaging device (generally called electronic cassette) that contains an indirect conversion type of radiation detector is in practical use for radiography. The indirect conversion type of radiation detector is provided with a scintillator and a sensor panel opposed to each other. The scintillator converts radiation e.g. X-rays into light. The sensor panel has a two-dimensional array of photoelectric converters, each for converting the light produced by the scintillator into an electric signal. There is also known a radiation detector having a reflective layer (refer to US Patent Application Publication No. 2008/0099694, for example). In this radiation detector, the sensor panel is disposed on one surface of the scintillator in a light exit direction, while the reflective layer is disposed on the other surface of the scintillator in a direction opposite to the light exit direction. The reflective layer reflects the light from the scintillator to the sensor panel, to make effective use of the light produced by the scintillator.
There is a type of scintillator that is composed of a plurality of columnar crystals erected along the light exit direction. The columnar crystals are manufactured by evaporation of cesium iodide (CsI) onto a substrate. This type of scintillator can reduce dispersion of the light produced in the scintillator, because the light propagates through the columnar crystals with total internal reflection, due to a light guide effect of the columnar crystals, heading for the sensor panel. Accordingly, the radiation imaging device using the scintillator with the columnar crystals can prevent deterioration of image sharpness, when producing an image by detecting the radiation.
In the scintillator having the columnar crystals, a gap is left between the adjacent columnar crystals to prevent the occurrence of crosstalk. The crosstalk is a phenomenon in which light propagating through one columnar crystal partly moves to another adjacent columnar crystal, when the two columnar crystals contact each other. The occurrence of the crosstalk causes a large deviation between a light emission point in the scintillator and a light incident point on the sensor panel, and results in a blur in a radiographic image.
The crosstalk occurs even if the columnar crystals are not in contact. As shown in FIG. 17, light 122 propagating through a columnar crystal 121a of a scintillator 120 with total internal reflection goes out of the columnar crystal 121a and enters an adjacent columnar crystal 121b, when an incident angle θx of the light 122 with respect to an interior wall of the columnar crystal 121a is less than a critical angle θx. When a refractive index of columnar crystals made of CsI is 1.8 and a refractive index of air present between the columnar crystals is 1.0, the critical angle θx is approximately 34°.
Moreover, the light 122 having entered the columnar crystal 121b from the columnar crystal 121a sometimes travels through further other columnar crystals, and reaches a columnar crystal far away from the columnar crystal 121a. This is because a gap between the columnar crystals is very small, and the light 122 having gone out of the columnar crystal 121a is hardly refracted. Especially, since a long wavelength component 122b of the light 122 is harder to refract than a short wavelength component 122a, an incident angle of the long wavelength component 122b on the columnar crystal 121b is maintained at a value less than the critical angle θx. Note that, the short wavelength component denotes light in a wavelength band of 620 nm or less in the case of a scintillator made of CsI:Tl, for example. The long wavelength component denotes light in a wavelength band over 620 nm.
The crosstalk caused by the incident angle, as described above, occurs not only in light propagating directly from the scintillator to the sensor panel, but also in light reflected from the reflective layer to the sensor panel. As shown in FIG. 18, most of the radiation incident upon the scintillator 120 is converted into the light at a radiation incident area, which is on a radiation incident side of the scintillator 120. The short wavelength component 122a of the light, having a relatively high refractive index, propagates through the columnar crystal 121a with the total internal reflection to the reflective layer 124. Even if the incident angle of the short wavelength component 122a with respect to the reflective layer 124 is less than the critical angle and the short wavelength component 122a enters the columnar crystal 121b from the columnar crystal 121a, the light incident point on the sensor panel 125 does not much deviate from the light emission point in the scintillator 120, because soon afterward the short wavelength component 122a has the incident angle equal to or more than the critical angle by refraction.
On the other hand, as shown in FIG. 19, since the long wavelength component 122b produced in the radiation incident area of a columnar crystal 121c of the scintillator 120 is harder to refract than the short wavelength component 122a, the long wavelength component 122b largely deviates from the light emission point before reaching the reflective layer 124. The long wavelength component 122b further deviates from the light emission point until being reflected from the reflective layer 124 and reaching the sensor panel 125, and is incident upon the sensor panel 125 at the light incident point far away from the light emission point. Note that, FIGS. 18 and 19 show just apart of the radiation detector, and the long wavelength component 122b does not necessarily propagate from one end of the sensor panel 125 to the other end thereof.
The indirect conversion type of radiation detector adopts either of an irradiation side sampling (ISS) method as shown in FIG. 18 and a penetration side sampling (PSS) method, which is not shown. In the ISS method, the sensor panel 125, the scintillator 120, and the reflective layer 124 are disposed in this order from the radiation incident side. The scintillator 120 converts the radiation that has been transmitted through the sensor panel 125 into light, and the sensor panel 125 detects the light. In the PSS method, a reflective layer, a scintillator, and a sensor panel are disposed in this order from the radiation incident side. The scintillator converts radiation that has been transmitted through the reflective layer into light. In the PSS method, the light produced in the radiation incident area of the scintillator is reflected from the reflective layer, which is disposed near the radiation incident area, to the sensor panel. On the other hand, in the ISS method, the light produced in the radiation incident area of the scintillator propagates to the reflective layer through the scintillator. The light is reflected from the reflective layer, and propagates through the scintillator to the sensor panel. Thus, a light propagation distance of the ISS method is twice as large as that of the PSS method, and the ISS method is more susceptible to the crosstalk.
The radiation detector disclosed in the U.S. Patent Application Publication No. 2008/0099694 has an absorbing layer provided between the scintillator and the reflective layer. The absorbing layer absorbs the long wavelength component of the light, for the purpose of preventing the blur of the radiographic image caused by the long wavelength component of the light produced by the scintillator. There is also known a radiation imaging device having the function of light reset (also called bias light emission, light calibration, or the like) in which reset light is applied from a light source to the photoelectric converters of the sensor panel. The application of the reset light improves deterioration in properties of the photoelectric converters due to an extended period of use, and stabilizes dark current occurring in the photoelectric converters (refer to Japanese Patent Laid-Open Publication No. 2007-147370). This function facilitates reducing deterioration in a motion radiographic image, for example. It is also known that infrared light is suitably used as the reset light (refer to U.S. Pat. No. 5,905,772, for example).
According to the radiation detector of the US Patent Application Publication No. 2008/0099694, provision of the absorbing layer to remove the long wavelength component causes high cost. Additionally, effective use of the long wavelength component of the light is desired, though the long wavelength component is unused at present.
In the above radiation detector of the ISS method, the reset light source cannot be situated on the sensor panel, because the sensor panel is disposed on the radiation incident side. To be more specific, if the light source is disposed on the sensor panel, the radiation is absorbed by the light source. Furthermore, if the light source is composed of an LED, an EL element, or the like, the light source sometimes emits the reset light involuntarily in response to application of the radiation. Since the radiation detector of the ISS method is provided with the reflective layer on the light exit side of the scintillator, the reset light source cannot be provided on the light exit side of the scintillator. In other words, if the reset light source is provided on the reflective layer, the reset light cannot be transmitted through the reflective layer and hence cannot be applied to the sensor panel.